To evaluate the etiology of poor children with obstructive sleep apnea syndrome (OSAS), we performed a dietary history, anthropometric measurements, polysomnography and measurements of oxygen consumption (VO2) during sleep in 9 children with OSAS before and after adenotonsillectomy. Post-operatively, the children had an increase in weight and a decrease in VO2. We conclude that children with OSAS gain weight following adenotonsillectomy due to a decrease in energy.